This invention relates in general to multiple-input-multiple-output (MIMO) wireless communication systems.
The governing equation of such MIMO systems is [1]y=Hx+n  (1)where, in complex base-band representation, H denotes the propagation complex channel matrix of R×L elements, with R and L the number of antenna elements at the (receiving) Right and (transmitting) Left sides respectively, where x is the transmitted complex base-band vector, and y is the received complex base-band vector distorted by the channel H and interfered by the receiver additive noise n.
The optimal solution to (1) above (e.g. [1]), of special relevance to our proposed invention, is the so called Maximum Likelihood Detection (MLD) scheme whereby the estimated transmitted vector xMLD is the vector which minimizes the (Euclidean) distance between the received vector y and the Channel transformed vector (H x), namely:xMLD=arg min∥y−H xi∥2  (2)where the minimization process takes place upon all the candidate vectors xi (not to be confused with the transmission vector elements xi). This scheme is known to be optimal in the sense of Symbol Error Rate (SER) vs. mean Signal to Noise Ratio (SNR) performance. However the exhaustive search MLD scheme is also known (again, [1]) to be of prohibitive complexity, the number of candidate vectors xi exponentially growing with both the dimension of x and the order of the symbol constellation; for example, the detection of a transmitted vector with M=4 sub-streams (e.g. a MIMO system comprising 4 radiating elements on each side) drawn out of 64-QAM constellation, would require 26×4 or approximately 16000000 metrics-calculations/vector, clearly prohibitive with present art computation devices technology.
Due to fore-mentioned MLD complexity some solutions to (1) consisting of simplifications to said exhaustive search method (2) have been proposed. One such popular proposed solution consists of a limited set search whereby the candidates search set is reduced to a ‘sphere’ of fixed or adaptive radius around the received vector y. This class of methods is sometimes appropriately named Sphere Decoding (SD for shorthand, e.g. [2]).
Still other simplified solutions to (1) have been proposed, like the QRD-M method ([3]) and the K-Best method ([4]) both consisting (like SD) of some sort of ‘volume’ or ‘tree’ exhaustive search. While substantially simplifying the full set exhaustive search as reflected in (2) above, all SD, QRD-M and K-Best however still suffer from poor, above cubic scalability (doubling H dimension multiplies the number of operations/vector by more than eight), constellation sensitivity (application of e.g. 64-QAM instead 16-QAM significantly affects complexity) and challenging Log Likelihood Ratios (LLRs) or soft decision results generation.
In the fore-cited and herein incorporated by reference cross-related patent applications    a. Nissani (Nissensohn), D. N., ‘Multi Input Multi Output Wireless Communication Reception Method and Apparatus’, U.S. Ser. No. 11/052,377, filed February 2005.    b. Nissani (Nissensohn), D. N., ‘Multi Input Multi Output Wireless Communication Reception Method and Apparatus’, U.S. Ser. No. 11/491,097, filed July 2006.    c. Nissani (Nissensohn), D. N., ‘Multi Input Multi Output Wireless Communication Reception Method and Apparatus’, PCT/IL07/00919, filed July 2007a fundamentally novel MIMO detection method was introduced along with its related apparatus. We hereon collectively denominate this method, described in the fore-cited Patent Applications, as Directional Lattice Descent (DLD in short). Its unique benefits include quadratic scalability (doubling H dimension multiplies complexity by approximately four), complexity insensitivity to modulating constellation, and seamless generation of LLRs or soft decision results.
In accordance with the said DLD invention the transmitted vector x of (1) above is treated as a point which belongs to a Transmission Lattice Bounded Region (e.g. [5]), said lattice informally defined as a countable infinite set of regularly spaced points in a multi-dimensional space, and said Transmission Lattice Bounded Region informally defined as a closed continuous region in this space (in typical constellations usually of hyper-cubic shape).
Still in additional accordance with the said DLD invention any noiseless received point H x of (1) above is perceived as belonging to a Reception Lattice Bounded Region, said Reception Lattice readily calculable from said Transmission Lattice by application of the channel matrix H.
Still in accordance with the said DLD invention, the MIMO reception process incurs the attempt of calculation of the MLD solution point xMLD of (2) above, that is a point which belongs to the Reception Lattice, which resides closest (in a Euclidean distance or other suitable metrics) to the received vector y of (1) and which is confined to the appropriate Reception Lattice Bounded Region.
In further accordance with the said DLD invention a vector detection process includes: the execution of an Initial Point Stage during which a Reception Lattice Initial Point is calculated; an iterative lattice Descent Stage during which, starting from said Reception Lattice Initial Point a possible descent is executed to sub-sequent Reception Lattice Points each calculated to reside at a smaller distance from the received vector y than its previous one, said Descent Stage terminated at a Reception Lattice Point for which no closer Reception Lattice Point to the received vector y is found (i.e. a probable minimal point); and an occasionally applied Outlier Stage during which, in cases whereby the Reception Lattice Point achieved by said Descent Stage is found to reside out of said Reception Lattice Bounded Region, another Reception Lattice Point is calculated typically by a re-conductance of said Descent Stage as above except that sub-sequent Reception Lattice Points are now constrained to the Reception Lattice Bounded Region.
The DLD method as briefly described above may be appropriately considered a discrete space equivalent of the well known gradient descent method of the ‘analog world’.
It is the goal of this patent application to introduce enhancements to the said DLD method of special importance at large MIMO dimensions and/or very high operation mean SNR. We refer the diligent reader to a thorough study of said fore-cited incorporated applications.